1. Field of the Invention
This invention is concerned with a process for converting synthesis gas, i.e., mixtures of gaseous carbon oxides with hydrogen or hydrogen donors, to hydrocarbon mixtures and oxygenates. More particularly, this invention is concerned with upgrading a C.sub.5.sup.+ fraction having an end-point of 340.degree. up to 400.degree. F obtained in a known Fischer-Tropsch synthesis process, so as to obtain a high yield of C.sub.5.sup.+ gasoline of enhanced octane and a low-pour, high diesel index fuel oil.
2. Other Prior Art
Processes for the conversion of coal and other hydrocarbons, such as natural gas to a gaseous mixture consisting essentially of hydrogen and carbon monoxide, or of hydrogen and carbon dioxide, or of hydrogen and carbon monoxide and carbon dioxide, are well known. Although various processes may be employed for the gasification, those of major importance depend either on the partial combustion of the fuel with an oxygen-containing gas or on a combination of these two reactions. An excellent summary of the art of gas manufacture, including synthesis gas, from solid and liquid fuels, is given in "Encyclopedia of Chemical Technology", edited by Kirk-Othmer, Second Edition, Volume 10, pages 353-433 (1966), Interscience Publishers, New York; the contents of which are herein incorporated by reference. The techniques for gasification of coal or other solid, liquid or gaseous fuel are not considered to be per se inventive here.
It is considered desirable to effectively and more efficiently convert synthesis gas, and thereby coal and natural gas, to highly valued hydrocarbons such as motor gasoline with high octane number, petrochemical feedstocks, liquefiable petroleum fuel gas, and aromatic hydrocarbons. It is well known that synthesis gas will undergo conversion to form reduction products of carbon monoxides, such as hydrocarbons at from about 300.degree. to about 850.degree. F under from about one to one thousand atmospheres pressure, over a fairly wide variety of catalysts. The Fischer-Tropsch process, for example, which has been most extensively studied, produces a range of products including liquid hydrocarbons, a portion of which have been used as low octane gasoline. The types of catalysts that have been studied for this and related processes include those based on metals or oxides of iron, cobalt, nickel, ruthenium, thorium, rhodium and osmium.
The wide range of catalysts and catalysts modifications disclosed in the art and an equally wide range of conversion conditions for the reduction of carbon monoxide by hydrogen provide considerable flexibility toward obtaining selected boiling-range products. Nonetheless, in spite of this flexibility it has not proven possible to make such selections so as to produce liquid hydrocarbons in the gasoline boiling range having clear research octane numbers above about 70 without further treatment. A review of the status of this art is given in "Carbon Monoxide Hydrogen Reactions", Encyclopedia of Chemical Technology, edited by Kirk-Othmer, Second Edition, Volume 4, pp. 446-488, Interscience Publishers, New York, the text of which is incorporated herein by reference.
Recently, a method for upgrading the C.sub.5.sup.+ liquid product of a Fischer-Tropsch synthesis having an end point from about 340.degree.-400.degree. F has been discovered, which method comprises pretreating the C.sub.5.sup.+ liquid product by hydrogenating it in the presence of a hydrogenation component (such as platinum or palladium) at conditions of temperature and pressure so as to selectively hydrogenate the diolefins contained in the C.sub.5.sup.+ liquid product and thereafter contacting the hydrogenated product, at a temperature within the range of about 575.degree. to 850.degree. F and at a pressure within the range of about atmospheric to 700 psig, with a crystalline aluminosilicate having certain well-defined characteristics. This method is described in a copending United States patent application, Ser. No. 684,511, filed May 7, 1976, now U.S. Pat. No. 4,052,477. It should be noted that the elevated temperatures of the step following pretreatment are such that most or substantially all of the hydrogenated product from the pretreatment step will be in the gaseous phase during the second, aluminosilicate contacting step.
Still more recently, another method for upgrading the C.sub.5+ liquid product of a Fischer-Tropsch synthesis having an end point from about 340.degree. to 400.degree. F has been discovered, which method comprises contacting the said liquid product in the absence of added hydrogen with a crystalline aluminosilicate, characterized by a pore dimension greater than about 5 Angstroms, a silica to alumina ratio of at least 12, and a constraint index within the range of 1 to 12 at dense phase process conditions of a temperature of about 400.degree. to 700.degree. F, a pressure of about 200 to 1000 psig, and a LHSV of about 0.1 to 2.0 and thereafter recovering a C.sub.5+ gasoline product boiling up to about 400.degree. F having an enhanced octane value, a 400.degree.-650.degree. F fuel oil product, and a 650.degree. F..sup.+ product. This method is described in copending United States patent application Ser. No. 732,235 filed Oct. 14, 1976. It should be noted that the C.sub.5+ liquid product is not fractionated prior to the aluminosilicate contacting step.